The present invention relates to a gas discharge laser having a discharge tube, in which a gas is present. The discharge tube has at least one aperture or window through which a laser beam emerges, or at which a laser beam is reflected. It may also be possible that the at least one aperture or window reflects a part of the laser beam and let through a part of the laser beam. At least one gas withdrawal point is provided for taking out a partial amount of the gas present within the gas discharge tube. The withdrawn amount of gas is guided through a filtering means, and is introduced in at least one gas inlet point in the zone of the aperture.
The invention further relates to a method of operating a gas discharge laser having a discharge tube, in which a gas is present, and which has at least one aperture through which a laser beam emerges, or at which a laser beam is reflected. At least a partial amount of the gas contained within the discharge tube is taken out, is cleaned by means of a filtering means and is fed in again in the zone of the at least one aperture. The cleaned gas flow may be fed in again in the zone of the at least one aperture in such a manner that a gas flow is formed directed away from the aperture. This can, for example, be achieved in that the cleaned gas flow will be directed towards the aperture, thereby rinsing the aperture free from possible dust particles and other adhering matter, whereupon the gas flow then is deflected into a direction directed away from the aperture. By this deflection of the gas flow, the penetration of dust particles from the interior of the discharge tube is avoided to a large extent, at least, however, impeded.
Finally, the invention relates to a novel use of a filtering means in the aforementioned technical field.
In gas discharge lasersxe2x80x94in particular excimer lasersxe2x80x94a gas is excited to emit light by means of an excitation and/or discharge reaction. The molecular or atomic compounds particularly suited for excimer lasers, which are also designated as gas or gases in the following for convenience, present energy transmissions by means of which electromagnetic radiation is preponderantly emitted in the ultraviolet spectral range. Therefore, excimer lasers nowadays represent the by far most intensive UV radiation sources. However, a high energy density is necessary for the excitation of the gases so as to generate stimulated emissions, which energy density is fed in in the form of an intensive pulsed electron beam or within the scope of a high-voltage discharge, since the effective cross-section for stimulated emission for the individual initial compounds within the gas is relatively small.
In the technical configuration of gas discharge lasers of this type, and in particular in excimer lasers, a particularly high effort in the constructional and set-up configuration has to be implemented for the components necessary for the targeted energy supply. Due to the very high energies which have to be fed in the excimer gas, impurities moreover occur in a system-contingent manner resulting, for example from local electrode fusing by compound reactions between the gas and the individual laser components. Said impurities, however, do not only affect the laser process as such, but are able to considerably reduce the beam outlet intensity of the laser due to deposits on the inner wall of the gas chamber enclosing the gas, and in particular in the zone of the initially mentioned apertures of the laser. Such deposits or dust particles hence can significantly affect the permeability of an outlet aperture or the reflection capability of a mirror. This problem increasingly occurs in lasers having a short wavelength, such as UV lasers.
From U.S. Pat. No. 4,534,034, a pumped gas discharge laser is believed to be known, wherein the gas present in the discharge tube is conveyed by means of a thereto connected but outside situated circulation pump through an electrostatic filter, so as to filter out dust particles etc. present in the gas. The cleaned gas flow is in each case fed into a zone of a aperture of the discharge tube after passing through a particular relaxation zone, so that the apertures possibly are kept clean by said gas flow, hence free from dust particles or other kinds of deposits. This solution may be subject to the problem that electrostatic dust collectors have a relatively large volume and require a separate voltage supply. Moreover, intricate relaxation zones here appear to be necessary in the gas conveyance system.
In U.S. Pat. No. 5,029,177, an excimer laser is believed to be disclosed, wherein likewise an electrostatic filter is provided for cleaning the gas in the discharge tube. For the transport of the gas flow through the electrostatic filter, the ventilator drum present in the discharge tube is used. The gas cleaned in the electrostatic filter flows in through an annular gap immediately in front of a aperture of the discharge tube. A pre-chamber in front of the apertures has a structure breaking the shock waves from the discharge, and impeding the gas flow and the movement of gas particles from the discharge towards the aperture.
From EP 0 669 047 B1, means are believed to be known for maintaining a clean laser aperture. Here, in particular, the structure of a pre-chamber in front of the aperture is described in more detail. The particular structure of the pre-chamberxe2x80x94as in the above-mentioned publicationxe2x80x94serves the purpose of creating a gas flow directed away from the aperture in the direction of the discharge, so as to impede the movement of dust into the pre-chamber in front of the aperture. Here, in addition, it is disclosed that apart from the gas present in the discharge tube, a second gas is used. To avoid an electric charge of the gases, an electrically grounded screen in front of the aperture is proposed.
It is an object of the invention to specify a technically simple solution for keeping apertures or windows in gas discharge lasers clean.
It is an other object of the invention to specify a cost efficient solution for keeping apertures or windows in gas discharge lasers clean.
It is an other object of the invention to specify a technically simple method to keep apertures or windows in gas discharge lasers clean.
According to one aspect of the present invention, this is achieved by means of a gas discharge laser comprising a discharge tube, in which a gas is present. This discharge tube has at least one aperture through which a laser beam emerges, and/or from which the laser beam is reflected. In the discharge tube, at least one gas withdrawal point is present for withdrawing a partial amount of the gas present in the discharge tube, and to feed it to a sintered filter for cleaning the withdrawn amount of gas. The sintered filter cleans the infed gas as far as possible from dust particles and such like. The cleaned gas flow is then fed in via at least one gas inlet point in the area of the at least one aperture, so that a dirtying of the aperture by dust particles present in the discharge tube may be prevented. Moreover, the respective aperture will probably be rinsed free from dust particles possibly adhering thereto.
In one exemplary embodiment of the present invention, the cleaned gas flow counteracts the pressure waves generated by the discharge, which transport dust particles away from the dischargexe2x80x94and inter alia towards the apertures.
The present invention may be based on the idea of using for the first time a sintered filter for cleaning a gas flow withdrawn from the discharge tube. As may be derived from the preceding explanations as to the documents mentioned above, electrostatic filters have always been used up to date for cleaning gases in technical devices of the kind of interest here. Surprisingly, however, it has turned out that sintered filters, despite their high flow resistance which has to be overcome by the gas flow, can also be used in gas discharge lasers of the initially mentioned kind. With sintered filters, a cost-efficient and maintenance-friendly alternative and completion, respectively, to the hitherto used electrostatic filters is for the first time given. In particular, the problems entailed by electrostatic filters may now be avoided or at least reduced.
In an other exemplary embodiment of the present invention a circulation means is present in the discharge tube, circulating the gas contained in the discharge tube and simultaneously conveying the gas flow from the withdrawal point through the sintered filter up to the gas inlet point. In other words: the entire circulation of the gas flow to be guided through the sintered filter may be performed by the circulation means present as such, and any additional and hence expensive pumping means therewith is probably not necessary. This configuration hence may have the advantage that the circulation means present as such in the discharge tube can also be used at the same time for generating the gas flow through the sintered filter, without complicated modifications of the circulation means becoming necessary. Namely, it has surprisingly turned out that the conveyance of a gas flow through a sintered filter affected as such by high flow resistance, is also possible with the means used up to date.
In an other exemplary embodiment of the present invention, one single gas withdrawal point is present for withdrawing gas from the discharge tube. From this single gas withdrawal point, several gas inlet points for several apertures are supplied. In this embodiment, hence only one withdrawal point is necessary in the discharge tube, however, a branched gas conduct system to the individual apertures might be necessary for this purpose. In this exemplary embodiment of the present invention, it is imaginable to provide a common sintered filter for all apertures. Yet, it is also possible to insert a sintered filter downstream of the branching of the common gas withdrawal point in each line.
In an other exemplary embodiment of the present invention, several gas withdrawal points are present for withdrawing gas from the discharge tube, and each withdrawal point is assigned to an aperture. In the associated gas conduct lines, a sintered filter is in each case additionally installed. This exemplary embodiment of the present invention can have the advantage that short line paths to the apertures suffice, whereby the line resistance is kept low, as well.
In an other exemplary embodiment of the present invention, the at least one gas withdrawal point for withdrawing gas from the discharge tube is present at the outlet of the circulation means. Thus, the kinetic energy of the circulated gas in the discharge tube is converted in probably an optimum manner into a gas conveyance flow in the direction of the sintered filter.
As has already been mentioned, in an exemplary embodiment of the present invention the circulation means preferably is a ventilator drum, which per se is present in a gas discharge laser such as an excimer laser.
In an other exemplary embodiment of the present invention, at least one gas withdrawal point for withdrawing gas from the discharge tube is arranged at the end of the ventilator drum, and the ventilator drum is longer in this zone than electrodes located at the discharge tube. Due to this arrangement, a gas conduct to the sintered filters and the respective apertures may be performed in an optimum manner, and the constructional effort is minimized.
In an other exemplary embodiment of the present invention, the gas withdrawal point is configured as a distributor tube made of a non-conductor material. In an alternative exemplary embodiment of the present invention, it is also imaginable that the distributor tube is arranged at an adequate distance from the components carrying high voltage. In this case, the material connection may not restricted to non-conducting materials.
So as to prevent major amounts of gas soiled with dust particles from reaching the respective apertures, it is preferred in an other exemplary embodiment of the present invention that a partition is arranged in front of the at least one aperture, which partition has, as compared to the tube cross-section of the discharge tube, a small opening interconnecting a pre-chamber formed by the partition in front of the aperture and the interior of the discharge tube. Into this pre-chamber debouches the gas inlet point.
Of course, it is moreover possible in other exemplar embodiments of the present invention to use in addition the means known per se, which possibly prevent dust from the discharge tube from reaching to the apertures.
In a further exemplary embodiment of the present invention the gas discharge laser is an excimer laser.
According to another aspect of the present invention, a method of operating a gas discharge laser, such as an excimer laser having a discharge tube, in which a gas is present, and which has at least one aperture or window through which a laser beam emerges and/or at which a laser beam is reflected. According to the inventive method, at least a partial amount of the gas contained in the discharge tube is cleaned in a sintered filter and is fed in again in the zone of the at least one aperture or window.
In a further exemplary embodiment of the present invention, the partial amount of gas cleaned in the sintered filter is fed in again in the zone of the at least one aperture or window in such a manner that a gas flow forms directed away from the aperture.
In a further exemplary embodiment of the present invention, a circulation means present in the discharge tube for the continuous circulation of the gas contained in the discharge tube is also used for conveying the gas flow withdrawn from the discharge tube through the sintered filter towards the aperture. Therewith, a separate circulation pump may be avoided, and the constructional effort for the use of the sintered filter may be kept low at the same time.
As has already been mentioned at the beginning, the use of a sintered filter for cleaning a gas flow withdrawn from the discharge tube of a gas discharge laser surprisingly is proposed for the first time, which gas flow is directed towards a aperture of the discharge tube for cleaning purposes.
In a further exemplary embodiment of the present invention a sintered filter can be used having pore sizes up to 10,000 nm and above. As the filter material of such sintered filters, for example, nickel, stainless steel or Hastelloy alloy are used.
In a further exemplary embodiment of the present invention, nickel is preferred from the group of materials mentioned above. Filters of that kind are, for example, produced by the company Mott of Connecticut, USA.
One skilled in the art will recognize that the terms xe2x80x9caperturexe2x80x9d and xe2x80x9cwindowxe2x80x9d define the same constructional part of a gas discharge laser. Hence, the use of one of the these terms does not limit the scope of protection of the present invention. Furthermore, the terms aperture and window comprise as such all embodiments known by on skilled in the art. In particular, these terms comprise windows, which are partly or fully transmissive, and which partly or fully reflect a laser beam. A window or aperture according to the present invention may also be defined as an optical element.